Processes and apparatus for making transversely drawn films with substantially uniaxial character

ABSTRACT

A process for stretching films is described. The process preferably stretches films in a uniaxial fashion. Preferably, optical films are stretched including multilayer optical films. Other aspects of the invention include a roll of stretched film and an apparatus for stretching films.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application Ser.No. 60/294,940, which is incorporated herein by reference.

This application is a divisional of U.S. Ser. No. 10/156,347, filed May28, 2002, the disclosure of which is herein incorporated by reference.

FIELD

The present invention relates to stretching films, particularly opticalfilms and more particularly to optical films that are to besubstantially uniaxially oriented. The present invention also comprisesan apparatus suitable for stretching such films and the resultant filmsstretched by the apparatus.

BACKGROUND

There are a variety of reasons to stretch films. PCT WO 00/29197discloses a method of biaxially stretching a polymeric film. The methodmay be used to impart mechanical characteristics to products such asfilm backing.

Stretching may enhance physical properties of crystalline plastic films.U.S. Pat. No. 2,998,772 discloses a machine for stretching film thatincludes circular discs that grasp edge portions of a film and stretchthe film transverse to a machine direction of the film.

FIG. 1 illustrates a conventional tenter drawing process that stretchescontinuously fed films transversely to the direction of film travel. Thefilm is gripped at both edges 2 by some gripping means, typically bytenter clips. The tenter clips are connected to tenter chains that ridealong linearly diverging tenter tracks or rails. This arrangementpropels the film forward in a machine direction of film travel andstretches the film. Thus an initial shape 4 in the film may be stretchedto the shape 6.

Tenter apparatus are described in U.S. Pat. Nos. 2,618,012, 3,502,766,3,890,421, 4,330,499; 4,525,317 and 4,853,602. Conventional tenterssuffer many drawbacks. The angle of divergence in conventional tentersis typically small, usually less than 10 degrees. Boundary trajectoriesreturn to a parallel, or nearly parallel, state prior to quenching thepolymeric film and slitting. Referring to FIG. 2, the unstretchedportion 4 of the film shown in FIG. 1 may have dimensions T, W and L.After the film is stretched by a factor of lambda (7), the dimensions ofthat portion of film have changed to those shown on portion 6. This isnot uniaxial stretch as described in greater detail below.

As used herein, the ratio of the final T′ to initial thickness of thefilm T (see FIG. 10) may be defined as the normal direction draw ratio(NDDR). The machine direction draw ratio (MDDR) may be defined as thelength of a portion of the film after stretching divided by the initiallength of that portion. For illustrative purposes only, see Y′/Y in FIG.11. The transverse direction draw ratio (TDDR) may be defined as thewidth of a portion of the film after stretching divided by the initialwidth of that portion. For illustrative purposes only, see X0/X in FIG.9.

The NDDR is roughly the reciprocal of the TDDR in a conventional tenter,while the MDDR is essentially unchanged. This asymmetry in MDDR and NDDRdraw causes differences in the various molecular, mechanical and opticalproperties of the film above and beyond the differences in propertiesbetween these directions and the stretch direction (TD). Illustrativeexamples of such properties include the crystal orientation andmorphology, thermal and hygroscopic expansions, the small strainanisotropic mechanical compliances, tear resistance, creep resistance,shrinkage, the refractive indices and absorption coefficients at variouswavelengths.

U.S. Pat. No. 4,862,564 discloses an apparatus for stretching athermoplastic material web. The device includes an exponential or othercurvilinear stretching profile. The apparatus provides a constant rateof stretch to the web, as opposed to the sharp peak and varying rate ofstretch provided with conventional straight course tenter apparatus.

Uniaxially drawn films have superior performance to simply monoaxiallydrawn films. For example, uniaxially drawn films are more easilyfibrillated or tom along the stretch direction (TD). In opticalapplications, matching the MD and ND indices of refraction is oftenadvantageous. For example, U.S. Pat. Nos. 5,882,774; 5,962,114; and5,965,247 (Jonza, et. al.) disclose materials with matched indexes ofrefraction for improved off-normal angle performance in brightnessenhancement applications of multilayer reflective polarizers.

FIG. 3 illustrates a known batch technique for stretching a multilayerfilm suitable for use as a component in an optical device such as apolarizer. The flat, initial film 3 is stretched uniaxially in thedirection of the arrows. The central portion necks down so that twoedges of the film are no longer parallel after the stretching process.Much of the stretched film 5 is unusable as an optical component. Only arelatively small central portion 9 of the film is suitable for use in anoptical component such as a polarizer. The yield and usable part sizefrom this process are small.

Japanese Unexamined Patent Publication Hei 5-11114 teaches thatcompensation films with matched MD and ND indices of refraction allowwider viewing angles in liquid crystalline displays.

A conventional method for attempting to make a uniaxially drawn film isto use a length orienter (L.O.) that draws the film longitudinally in MDacross at least one span between rollers of differing speed. The MDDRimparted along this span or draw gap is essentially the ratio of thespeed of the downstream roll to the upstream roll. Because the filmfreely spans the rollers without edge constraints, the film can neckdown in width as well as thin in caliper as it draws. Thus the TDDR canbe reduced substantially below unity and can possibly be made to equalthe NDDR. The method is fraught with difficulties and limitations. Onedisadvantage is the limitation on part size. An initial web of givenwidth is reduced in width by a factor of the square root of thereciprocal of MDDR. Thus a final film made with an L.O. has asubstantially reduced width. When contrasted to a film made by a tenter,which increases the width by roughly the TDDR (excluding edge lossesfrom gripping), the L.O. under uniaxial conditions reduces the possiblepart size substantially.

Stretching longitudinally tends to amplify machine direction propagatedcaliper imperfections such as die lines. In order to achieve a highdegree of uniaxial character, the L.O. needs a long span relative to thefilm initial width. Practically, this requires a large device and longfilm spans that may be hard to control.

Japanese Unexamined Patent Publication Hei 6-34815 points out anotherlimitation of making films for optical applications over rollers. Thisdocument points out that rollers can scratch or otherwise damage thesurface of the film. Films with delicate coatings or with soft skinlayers could be easily deleteriously impacted.

In Japanese Unexamined Patent Publication Hei-150115, the effectiveinitial width is reduced by introducing MD oriented slits into the filmin a periodic fashion. This method even more severely limits theavailable part width.

There have been many attempts to draw films in a uniaxial fashion.Japanese Unexamined Patent Publication Nos. Hei 5-288931, 5-288932,6-27321 and 6-34815 (H. Field, et. al.) describe methods where film isfed into clips whose gripping surfaces form an out-of-plane waveform.Since the actual contour length along MD of the film is much longer thanthe in-plane projection of that contour length along MD of the tenter,the actual rate of film fed in is higher than its planar projection. Thefilm is initially fed in a similar out-of-plane waveform (e.g. it iscorrugated). The method makes use of the MD tension that develops duringdraw to take up the slack of the corrugation and flatten the final film.In a variation, the film is drawn normally and then placed in thewaveform clips. Heat treatment under tension after draw and theresulting shrinkage forces are then relied on to flatten the web. Themethod is described in conjunction with polysulfone films at low levelsof overfeed (under 20%). The method is likely limited by process issuessuch as the draw ratio range required and heat transfer. Many usefuluniaxially oriented films require draw ratios in excess of 4. These inturn would require overfeeds in excess of 100%, resulting in deepout-of-plane folds that would be difficult to heat uniformly. Forexample, the heat transfer to the tops and bottoms of the folds could bemuch higher than in the center plane due to the closer proximity to theheating plenums. This would tend to limit line speeds. Such large foldscould also collapse and stick to each other as the web strength weakenedin the pre-heat needed to effect draw, thereby causing the method tofail. At low levels of overfeed, the method reports good flatteningacross the film. As the boundary waveform became deeper, it is believedthat the yield and quality of the final film would be adverselyimpacted.

Japanese Unexamined Patent Publications, Hei 5-241021, 6-51116 and6-51119 disclose clip gripping surfaces remaining in-plane during draw.The film is fed into the clips at an out-of-plane angle while the clipsare moving around an out-of-plane radius. The out-of-plane radiuscreates a temporary increase in the separation between the individualclips. After rounding the curve, the clip gripping surfaces returnin-plane, the clips remain separated but more closely spaced, andcorrugated portions of the film provide extra slack lie between theclips. The method relies on the tension during draw to flatten the filmin-plane. The method may suffer the disadvantages of large corrugationsfor high draw ratio conditions. Additionally, since the clips remainseparated prior to draw, the edges of the film forming the initialcorrugations are unsupported. As the drawing proceeds and stressesbuild, these unsupported edges begin to pull inwards towards the filmcenterline. Eventually large scallops form between the clips. Thescallops not only make the edges unusable, but also create large calipervariations through the film. This adversely impacts the yield andquality of the final film.

Japanese Unexamined Patent Publications, Hei 5-11113 disclosesdecoupling the MD line speed from the instantaneous film MD velocity bymaking the process partly discontinuous in mass flow. Transverselyoriented slits are introduced into the web. These allow central portionsof the otherwise continuous film to pull away from each other, allegedlyallowing more substantially uniaxially drawn material in these portions.This method puts severe limits on usable part size and yield.

U.S. Pat. No. 4,349,500 (Yazawa, et. al.) discloses a film fed betweentwo rotating disks or wheels. The film is gripped by two continuousbelts. The film and the disks all lie in the same plane. The filmstretches transversely between the counter rotating disks as its edgesfollow the diverging circumferential edges of the disks. The divergenceangle of the draw becomes large, and the MD velocity of the film slowsby the cosine of this divergence angle. The belt speed remains constant.In this manner, the output velocity is reduced from the input velocityof the film. The film is released from its gripping belts and the filmis taken up at the slower MD velocity.

The method discloses the adjustment of the separation distance betweenthe centers of rotation of the disks and the size of the disks. Onedisadvantage of this method, discussed in U.S. Pat. No. 5,826,314, isthe difficulty of maintaining good gripping of the film with the beltsystem. This would be particularly challenging in the stretching offilms that develop high levels of drawing stress, e.g. polyesters drawnnear their glass transition temperatures. It is believed that manymaterials used in this process would acquire a wrinkle or anon-uniaxially drawn permanent set using this method. For example,polyesters monoaxially drawn near their glass transitions while holdingtheir MD lengths fixed may wrinkle rather than snap back in-plane whenthe final width is reduced in a succeeding step towards that anticipatedfor the substantially uniaxial case. Wrinkling also can occur when theMD reduction is applied too late in the TD drawing step.

Swenson U.S. Pat. No. 5,043,036 describes a canted wheel film drawingapparatus. Here the disks are no longer in-plane with the film and thusthe sheet is stretched between out-of-plane boundary trajectoriesdefined by the circumferences of the canted wheels. The method isdescribed as a means of stretching films comprising elastomeric layers.As pointed out in U.S. Pat. No. 3,807,004, due to the developing MDtension along the progress of the draw, stretching between suchout-of-plane curved surfaces causes the film surface to become saddleshaped. The central portion of the film straightens out as it is notdirectly held, as is the film at the boundary trajectories, and thus itdraws along a different path than the edges. This non-uniform drawingcan result in significant caliper and property variations across theweb, and is a major disadvantage for drawing films along boundarytrajectories that move out-of-plane.

U.S. Pat. No. 3,807,004 described a variety of methods for partiallydealing with the saddle formation. Profiling the initial film thicknessor temperature distribution is suggested as a means to uniform caliper,although property variations due to different drawing histories wouldremain. Alternatively, a support device could force the film in thecentral portion to conform to the curved out-of-plane trajectory.Friction and concomitant damage to the film surface might be reduced byvarious methods including an air cushion. Saddling also manifests invarious operations with the aforementioned disk orienter as described inU.S. Pat. No. 4,434,128. A convex guide surface is used to counter thesaddling. Damage to the film surface from the application of suchmethods is another disadvantage to the method. In particular, films usedin optical applications are particularly sensitive to surface defects asmay be caused by scuffing and other contact-related defects.

SUMMARY

The present invention comprises processes for stretching film to providedesirable properties (e.g. optical properties), films stretchedaccording to such processes and apparatus for stretching films. Inpreferred embodiments, the invention addresses shortcomings of the priorart such as excessive thickness deviation across the width of desireduse of the final film, excessive anisotropic property deviation fromfiber symmetry across the width of desired use of the final film,wrinkles and other non-flat imperfections in the final film, and surfacecontacting that can cause surface damage to the final film.

In one aspect, the present invention includes a process for forming anoptical film with predetermined optical properties, including the stepsof providing a multilayer film having alternating layers of polymericmaterials with predetermined optical properties, such that the film isdefined in reference to a coordinate system of first and secondorthogonal in-plane axes and a third mutually orthogonal axis in athickness direction of the film; feeding the multilayer film to astretcher; stretching the film along the first in-plane axis of the filmwith the stretcher while allowing contraction of the film in the secondin-plane axis and in the thickness direction of the film, with thestretching achieved by grasping edge portions of the film and moving theedge portions of the film along predetermined paths which diverge tocreate substantially the same proportional dimensional changes in thesecond in-plane axis of the film and in the thickness direction of thefilm.

In one embodiment, the predetermined paths are shaped so as to createsubstantially the same proportional dimensional changes in the secondin-plane axis of the film and in the thickness direction of the film. Ina preferred embodiment, at least one of the edge portions of the film ismoved along a predetermined path that is substantially parabolic.

In a different embodiment, the speed of the edge of the film iscontrolled to create substantially the same proportional dimensionalchanges in the second in-plane axis of the film and in the thicknessdirection of the film.

In another embodiment, at least one of the edge portions of the film ismoved along a predetermined path at a substantially constant speed.

In a preferred embodiment, the process is a continuous process and thefilm is fed continuously to the stretcher. The film may be fedcontinuously to the stretcher from a roll, or the film may be extrudedor coextruded in-line with the stretcher.

In another embodiment, the strain rate along the stretch direction ofthe first in-plane axis is not constant during at least a portion of thestretch.

Preferably, the proportional dimensional changes in the second in-planeaxis of the film and in the thickness direction of the film aresubstantially the same throughout substantially all of the draw history.

In another embodiment, the edge portions of the film move alongpredetermined paths that lie substantially within a plane defined by thefirst and second in-plane axes.

In yet another embodiment, the edge portions of the film move along apredetermined path that is three-dimensional.

Preferably, the edge portions of the film move along predetermined pathsthat are substantially symmetrical about a center axis.

More preferably, the film has first and second major surfaces and thefilm is stretched without physically contacting the first and secondmajor surfaces of the film except at the edge portions of the film.

In another aspect, the present invention includes a process for formingfilm with predetermined properties, including the steps of providing afilm that is defined in reference to a coordinate system of first andsecond orthogonal in-plane axes and a third mutually orthogonal axis ina thickness direction of the film; feeding the film to a stretcher;stretching the film along the first in-plane axis of the film with thestretcher while allowing contraction of the film in the second in-planeaxis and in the thickness direction of the film, with the stretchingaccomplished by grasping edge portions of the film and moving edgeportions of the film along predetermined paths that are shaped to createsubstantially the same proportional dimensional changes in the secondin-plane axis of the film and in the thickness direction of the filmthroughout substantially all of the stretching step.

In another aspect, the present invention includes a process for formingfilm with predetermined properties, including the steps of providing afilm that is defined in reference to a coordinate system similar to thatdescribed above; feeding the film to a stretcher in a direction oftravel of the film; stretching the film along the first in-plane axis ofthe film with the stretcher while allowing contraction of the film inthe second in-plane axis and in the thickness direction of the film,with the stretching accomplished by grasping edge portions of the filmand moving the edge portions of the film along substantially parabolicpaths that diverge.

In a preferred embodiment, the present invention includes a continuousprocess for forming film with predetermined properties, including thesteps of providing a film that is defined in reference to a coordinatesystem similar to that described above; continuously feeding the film toa stretcher in a direction of travel of the film; stretching the filmalong the first in-plane axis of the film with the stretcher whileallowing contraction of the film in the second in-plane axis and in thethickness direction of the film, with the stretching accomplished bygrasping edge portions of the film and moving the edge portions of thefilm along predetermined paths that diverge in such a way that thestrain rate in the direction of the first in-plane axis is not constantduring at least a portion of the stretching step.

In another aspect, the present invention includes a roll of optical filmwith predetermined optical properties defined in reference to acoordinate system of first and second orthogonal in-plane axes and athird mutually orthogonal axis in a thickness direction of the film,such that the roll of optical film is constructed by the process ofcontinuously feeding a roll of film to a stretcher and continuouslystretching the film along the first in-plane axis of the film with thestretcher while allowing contraction of the film in the second in-planeaxis and in the thickness direction of the film to create substantiallythe same proportional dimensional changes in the second in-plane axis ofthe film and in the thickness direction of the film.

Preferably, the roll of optical film is a multilayer optical film havingalternating layers of polymeric materials having predetermined opticalproperties.

More preferably, the roll of optical film has portions suitable forbeing incorporated into a polarizer. Even more preferably, the polarizermay be a reflective polarizer.

Preferably, the roll of film is constructed by the process of stretchingthe film so that substantially the same proportional dimensional changesin the second in-plane axis of the film and in the thickness directionof the film are created throughout substantially all of the stretchingprocess.

In yet another aspect, the present invention is a stretcher forcontinuously processing film, including a means for receiving acontinuous supply of film having predetermined properties, with the filmbeing defined in reference to a coordinate system as described above;clamping means for grasping edge portions of the film; and stretchingmeans for continuously moving the clamping means along predeterminedpaths that diverge so that the film is stretched along the transversedirection while allowing contraction of the film in the machinedirection and the thickness direction, with the predetermined pathshaving shapes that are selected to create substantially the sameproportional dimensional changes in the machine direction of the filmand in the thickness direction of the film to impart predeterminedoptical properties into the film.

The stretcher preferably includes a means for receiving a supply of thefilm which includes a means for receiving the film from a roll of saidfilm.

The stretcher also preferably includes take away means for removing thestretched film from the stretcher. In a preferred embodiment, the takeaway means includes means for severing the stretched film from rapidlydiverging edge portions of the film and moving the stretched portion outof the stretcher.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be more completely understood in the followingdetailed description of various embodiment of the invention inconnection with the accompanying drawings, in which:

FIG. 1 is a schematic top view of a prior art tenter apparatus used tostretch film;

FIG. 2 is a perspective view of a portion of film in the prior artprocess depicted in FIG. 1 both before and after the stretching process;

FIG. 3 is a schematic illustration of a prior art batch process fordrawing a multilayer optical film showing the film both before and afterthe stretch;

FIG. 4 is a block diagram showing steps according to one aspect of thepresent invention;

FIG. 5 is a schematic illustration of the stretching process accordingto a preferred embodiment of the present invention;

FIG. 6 is a perspective view of a portion of film in the processdepicted in FIG. 5 both before and after the stretching process;

FIG. 7 is a schematic top view of a portion of a stretching apparatusaccording to one aspect of the present invention;

FIG. 8 is an end view of the apparatus of FIG. 7;

FIG. 9 is a schematic view of a stretched film illustrating a coordinateaxis showing a machine direction (MD), a normal direction (ND) atransverse direction, an initial width X, a stretched width XO and aboundary trajectory IBT;

FIG. 10 is a side view of a stretched film illustrating an initialthickness T, a final thickness T′ a the normal direction ND;

FIG. 11 is a schematic view of a stretched film illustrating acoordinate axis showing a machine direction (MD), a normal direction(ND) a transverse direction (TD), an initial length Y, a stretchedlength Y′ and a boundary trajectory IBT; and

FIG. 12 is a perspective view of a take away portion of an apparatusaccording to an aspect of the present invention.

The invention is amenable to various modifications and alternativeforms. Specifics of the invention are shown in the drawings by way ofexample only. The intention is not to limit the invention to theparticular embodiments described. Instead, the intention is to cover allmodifications, equivalents, and alternatives falling within the spiritand scope of the invention as defined in the claims.

DETAILED DESCRIPTION

FIG. 4 is a block diagram of a process according to an aspect of thepresent invention. The process forms a film with predeterminedproperties.

The present invention is applicable generally to a number of differentfilms, materials and processes. The present invention is believed to beparticularly suited to fabrication of polymeric optical films where thevisco-elastic characteristics of materials used in the film areexploited to control the amount, if any, of molecular orientationinduced in the materials when the film is drawn during processing. Asdescribed below, consideration of the various properties of thematerials used to produce optical films may be exploited to improve theoptical films. The improvements include one or more of improved opticalperformance, increased resistance to fracture or tear, enhanceddimensional stability, better processability and the like.

A variety of optical films may be stretched or drawn according to thepresent invention. The films may comprise single or multi-layer films.Suitable films are disclosed, for example, in U.S. Pat. Nos. 5,699,188;5,825,543; 5,882,574; 5,965,247; 6,096,375; and PCT Publication Nos. WO95/17303; WO 96/19347; WO 99/36812; WO 99/36248 (the entire contents ofeach of which are herein incorporated by reference).

Films made in accordance with the present invention may be useful for awide variety of products including polarizers, reflective polarizers,dichroic polarizers, aligned reflective/dichroic polarizers, absorbingpolarizers, retarders (including z-axis retarders).

The films may comprise the optical element itself or they can be used asa component in an optical element such as matched z-index polarizersused in beamsplitters for front and rear projection systems, or as abrightness enhancement film used in a display or microdi splay. Itshould also be noted that the stretcher described below in accordancewith the present invention may be used with a length orienter to make amirror from a multi-layer optical film.

A process for fabricating an optical film in accordance with oneparticular embodiment of the present invention will be described withreference to FIGS. 9, 10 and 11. These figures illustrate a portion ofan optical film. The depicted optical film may be described withreference to three mutually orthogonal axes TD, MD and ND. In theillustrated embodiment, two orthogonal axes TD and MD are in the planeof the film (in-plane axes) and a third axis extends in the direction ofthe film thickness.

FIG. 4 is a block diagram of a process according to the presentinvention. In step 30, the film is supplied or provided to an apparatusfor stretching the film. The process may optionally include apreconditioning step 32. The film is stretched in step 34. The film mayoptionally be post-conditioned in step 36. The film is removed from thestretching apparatus in step 38.

FIG. 5 illustrates a preferred embodiment of the invention. The processincludes the step of providing a film 40 to a stretching apparatus (seeregion 30′). As shown in FIGS. 9, 10 and 11, the film may be referred towith reference to a coordinate system of first and second orthogonalin-plane axes (e.g. machine direction MD and transverse direction TD)and a third mutually orthogonal axis in a thickness direction of thefilm (e.g. normal direction ND).

The process includes the steps of feeding the film 40 to a stretcher(see region 30′); stretching the film along the first in-plane axis ofthe film with the stretcher while allowing contraction of the film inthe second in-plane axis and in the thickness direction of the film (seeregion 34′), with the stretching achieved by grasping edge portions ofthe film and moving the edge portions of the film along predeterminedpaths 64 which diverge to create substantially the same proportionaldimensional changes in the second in-plane axis of the film and in thethickness direction of the film.

The process may optionally include a preconditioning step (see region32′) such as providing an oven 54 or other apparatus. Thepreconditioning step may include a preheating zone (see region 42 of thefilm) and a heat soak zone (see region 44).

The film is stretched in region 34′. Edges of the film may be grasped bymechanical clips that are moved by rollers 62 in the direction of thearrows. In a preferred embodiment, paths 64 are parabolic orsubstantially parabolic.

The process includes an optional post-conditioning step (see region36′). For example, the film may be set in region 48 and quenched inregion 50. A belt and rollers may optionally be used to progress thefilm in this region. A cut may be made at 58 and flash or unusableportion 56 may be discarded.

To maintain a substantially uniaxial draw throughout substantially allof the draw history (as shown in FIG. 5), at the end of the transversestretch, the rapidly diverging edge portions 56 are preferably severedfrom the stretched film 48 at a slitting point 58.

Release of the selvages from a continuous gripping mechanism can be donecontinuously; however, release from discrete gripping mechanisms, suchas tenter clips, should be done over discrete MD section of the film,e.g. all the material under any given clip is released at once. Thisdiscrete release mechanism may cause larger upsets in stress that may befelt by the drawing web upstream. In order to assist the action of theisolating takeaway device, it is preferred to use a continuous selvageseparation mechanism in the device, e.g. the “hot” slitting of theselvage from the central portion of a heated, drawn film.

The slitting location is preferably located near enough to the“gripline”, e.g. the isolating takeaway point of first effective grippercontact, to minimize stress upsets upstream of that point. If the filmis slit before the gripping, instable takeaway can result, e.g. by film“snapback” along TD. The film is thus preferably slit at or downstreamof the gripline. Slitting is a fracture process and, as such, typicallyhas a small but natural variation in spatial location. Thus it may bepreferred to slit slightly downstream of the gripline to prevent anytemporal variations in slitting from occurring upstream of the gripline.If the film is slit substantially downstream from the gripline, the filmbetween the takeaway and boundary trajectory will continue to stretchalong TD. Since only this portion of the film is now drawing, it nowdraws at an amplified draw ratio relative to the boundary trajectory,creating further stress upsets that could propagate upstream, e.g.undesirable levels of machine direction tension propagating upstream.

The slitting is preferably mobile and re-positionable so that it canvary with the changes in takeaway positions needed to accommodatevariable final transverse draw direction ratio. An advantage of thistype of slitting system is that the draw ratio can be adjusted whilemaintaining the draw profile simply by moving the take-away slittingpoint 58.

A variety of slitting techniques may be used included a heat razor, ahot wire, a laser, a focused beam of intense IR radiation or a focusedjet of heated air. In the case of the heated jet of air, the air may besufficiently hotter in the jet to blow a hole in the film, e.g. by heatsoftening, melting and controlled fracture under the jet. Alternatively,the heated jet may merely soften a focused section of the filmsufficiently to localize further drawing imposed by the still divergingboundary trajectories, thus causing eventual fracture downstream alongthis heated line through the action of continued film extension. Thefocused jet approach may be preferred in some cases, especially when theexhaust air can be actively removed, e.g. by a vacuum exhaust, in acontrolled fashion to prevent stray temperature currents from upsettingthe uniformity of the drawing process. For example, a concentric exhaustring around the jet nozzle may be used. Alternatively, an exhaustunderneath the jet, e.g. on the other side of the film, may be used. Theexhaust may be further offset or supplemented downstream to furtherreduce stray flows upstream into the drawing zone.

The process also includes a removal portion in region 38′. Optionally aroller 65 may be used to advance the film, but this may be eliminated.Preferably the roller 65 is not used as it would contact the stretchedfilm 52 with the attendant potential to damage the stretched film.Another cut 60 may be made and unused portion 61 may be discarded.

FIG. 6 helps illustrate what is meant in this application when it issaid that the process “creates substantially the same proportionaldimensional changes in the second in-plane axis of the film and in thethickness direction of the film”. Three dimensional element 24represents an unstretched portion of film (see e.g. FIGS. 5 and 6) withdimensions T, W and L. Three dimensional element 26 represents element24 after it has been stretched a length lambda. As can be seen in FIG.6, the thickness and width have been reduced by the same proportionaldimensional changes. FIG. 6 represents a uniaxial stretch, as opposed,for example, to the non-uniaxial stretch shown in FIG. 2.

The present invention is not limited to perfect uniaxial stretching.Instead, the present invention includes processes, apparatus and filmsthat are “substantially” uniaxially stretched. The following discussionand observations are provided to define what is within the scope of thepresent invention.

“Substantially” uniaxially drawn films preferably possess fiber symmetryin which the properties in MD and ND are similar within a given materiallayer (as films comprising multiple layers may not themselves possessfiber symmetry due to the layered natured of the film composite). Thismay exist in an elastic material when two of the draw ratios are equal.When one of the directions, e.g. TD, is stretched, then the other twodirections, e.g. MD and ND, preferably have equal draw ratios. Assumingvolume conservation, the MDDR and NDDR both should approach the squareroot of the reciprocal of the TDDR. Films drawn in a conventional tenterare not substantially uniaxially drawn even though they have beenphysically drawn in only one direction (so-called “monoaxial” drawing)because the boundary constraints of the process impart differencesbetween MDDR and NDDR.

The present invention is also not limited to those processes thatstretch film under uniaxial conditions throughout the entire history ofthe stretch or draw. In a preferred embodiment, the present inventionaddresses the inadequacy of prior art processes (e.g. the diskorienters) to provide the substantially uniaxial constraint on machinedirection draw ratio (MDDR) and transverse direction draw ratio (TDDR)throughout the entire history of the draw. The failure of the prior artto provide the uniaxial condition throughout the draw is a cause ofwrinkling and other out-of-plane defects in the final film.

In a preferred embodiment, the present invention provides a process inwhich a substantially uniaxial draw is provided via the boundarytrajectories throughout the drawing step. More preferably, the processprovides this history dependence while maintaining the film in-plane.However, the stretching step need not be performed within asubstantially planar region (as depicted in FIG. 5). As discussed inmore detail below, it is within the present invention to provide aboundary trajectory of the film that is three dimensional andsubstantially non-planar.

Preferably the present invention maintains the deviation from a uniaxialdraw within certain tolerances throughout the various portions of thedrawing step. Optionally, the present invention may maintain theseconditions while deforming a portion of the film out-of-plane in aninitial portion of the draw, but return the film in-plane during a finalportion of the draw.

In a uniaxial transverse draw maintained throughout the entire historyof the draw, the instantaneous MDDR equals the square root of thereciprocal of the TDDR. As discussed above in conjunction with apreferred embodiment of the present invention, the film may be drawnout-of-plane using out-of-plane boundary trajectories, i.e. boundarytrajectories that do not lie in a single Euclidean plane. There areinnumerable, but nevertheless particular, boundary trajectories meetingrelational requirements of this preferred embodiment of the presentinvention, so that a substantially uniaxial draw history may bemaintained using out-of-plane boundary trajectories.

The boundaries may be symmetrical, forming mirror images through acentral plane, e.g. a plane comprising the initial center point betweenthe boundary trajectories, the initial direction of film travel and theinitial normal to the unstretched film surface. In this preferredembodiment the film may be drawn between the boundary trajectories alonga cylindrical space manifold formed by the set of line segments ofshortest distance between the two opposing boundary trajectories as onetravels along these boundary trajectories at equal rates of speed fromsimilar initial positions, i.e., colinear with each other and theinitial center point. The trace of this ideal manifold on the centralplane thus traces out the path of the film center for an ideal draw. Theratio of the distance along this manifold from the boundary trajectoryto this central trace on the central plane to the original distance fromthe start of the boundary trajectory to the initial center point is theinstantaneous nominal TDDR across the film spanning the boundarytrajectories, i.e. the ratios of the half-distances between the currentopposing points on the boundary trajectories and the half-distancesbetween the initial positions of the opposing points on the boundarytrajectories. As two opposing points move at constant and identicalspeeds along the opposing boundary trajectories, the correspondingcenter point on the central trace changes speed as measured along thearc of the central trace, i.e. the curvilinear MD. In particular, thecentral trace changes in proportion with the projection of the unittangent of the boundary trajectory on the unit tangent of the centraltrace.

Uniaxial draw may be maintained along the entire history of the draw aslong as the speed of the central point reduces at every point along thecentral trace from its initial speed by a factor of exactly the squareroot of the reciprocal of the instantaneous TDDR measured between thecorresponding opposing points on the opposing boundary trajectories.This is the uniaxial condition when viewing the instantaneous MDDR of adifferential film arc along the idealized central trace. The uniaxialcondition may be achieved by controlling the ratio of the instantaneousrate of change of arc length along the central trace to theinstantaneous rate of change of arc length at a corresponding opposingpoint on a boundary trajectory, i.e. the curvilinear MDDR. Bymaintaining this constraint, suitable boundary trajectories within thegeneral class of three dimensional space curves may be found andspecified within the context of this preferred embodiment of the presentinvention.

Preferably, the film is drawn in plane such as shown in FIG. 5. Thisavoids the problems of maintaining the central portions of the film tothe idealized space manifold of draw. The design of the boundarytrajectories is also simplified because the in-plane constraint reducesthe number of variables. There is one pair of mirror image opposingboundary trajectories that maintains the uniaxial condition throughoutthe process of the draw. The design of the boundary trajectory mayproceed by considering the instantaneous in-plane draw ratios MDDR andTDDR rather than the MDDR or MD speed as defined along the curvilinearcentral trace. The result is a pair of mirror symmetric in-planeparabolic trajectories diverging away from the in-plane MD centerlinebetween them. The parabola may be portrayed by first defining the TD asthe “x” direction and the MD as the “y” direction. The MD centerlinebetween the opposing bounding parabolas may be taken as the y coordinateaxis. The coordinate origin may be chosen as the initial centerpoint ofthe central trace between the parabolic trajectories and the left andright bounding parabolas are chosen to start at minus and plus X₀,respectively, where y=0. The right bounding parabolic trajectory, forpositive y values, that embodies this preferred embodiment of theinvention is (equation 1):x/x ₀=(¼) (y/x ₀)²+1The left bounding parabolic trajectory is obtained by multiplying theleft-hand side of the above equation 1 by minus unity.

The parabolic trajectory of equation 1 provides the uniaxial condition.As such it represents the in-plane drawing state in which MD tensionshould be negligible. In order to obtain good yield and propertyuniformity across the usable width of the final film, the principal axesof molecular orientation and of the resulting properties as induced bythe draw preferably remain nearly constant. In this case, straight linesdrawn along TD, the principal draw direction, remain substantiallystraight after drawing. In tenter processing of biaxially orientedfilms, this is typically not the case.

It should be again noted that the present invention is not limited toperfectly uniaxially drawn films. In practice, nearly or “substantially”uniaxially drawn films are sufficient to make components of sufficientdesired properties. Often, the uniformity of such films is moreimportant than the precise manifestation of uniaxial character. Adiscrepancy in uniaxial character in properties such as refractive indexis tolerable in many applications. For example, the off-anglecharacteristics of reflective polarizers used in liquid crystallinedisplay applications is strongly impacted by the difference in the MDand ND indices of refraction when TD is the principal mono-axial drawdirection. An index difference in MD and ND at 633 nm of 0.08 may beacceptable in some applications. A difference of 0.04 is allowable inothers. In more stringent applications, a difference of 0.02 or less ispreferred. Thus preferred embodiments of films according to the presentinvention include the class of nearly or substantially uniaxially drawnfilms, processes for creating such substantially uniaxially drawn filmsand apparatus for creating such substantially uniaxially drawn films.

A preferred method for calculating trajectories within a specifiedenvelope of nearly or substantially uniaxial character is discussed. Themethod determines the “right” boundary trajectory directly, and the“left” boundary trajectory is taken as a mirror image. First, theenvelope constraint is set by defining an instantaneous functionalrelationship between TDDR measured between the opposing boundarytrajectories and the MDDR defined as the cosine of the non-negativedivergence angle of those boundary trajectories, over a chosen range ofTDDR. Next, the geometry of the problem is defined as described in thediscussion of the parabolic trajectories. X₁ is defined as the initialhalf width between the boundary trajectories and ratio (x/x₁) isidentified as the instantaneous TDDR, where x is the current x positionof an opposing point on the boundary trajectory. Next, the instantaneousfunctional relationship between the TDDR and MDDR is converted to arelationship between TDDR and the divergence angle. Next, the boundarytrajectory is constrained to satisfy the differential equation, equation2:d (x/x ₁)/d (y/x ₁)=tan(θ)where tan(θ) is the tangent of the divergence angle θ, and y is the ycoordinate of the current position of the opposing point on the rightboundary trajectory corresponding to the given x coordinate. Next, thedifferential equation may be solved, e.g. by integrating 1/tan(θ) alongthe history of the TDDR, (x/x₁), from unity to the maximum desired valueto obtain the complete coordinate set {(x,y)} of the right boundarytrajectory, either analytically or numerically. The divergence angle isthe non-negative, smallest angle made between the direction of travel ofthe centerline of the film and the instantaneous boundary trajectory at(x,y). In the symmetric, in-plane case of equation 2, the travel of thecenter line is along MD, e.g. the divergence angle is zero, when theboundary trajectories are parallel to MD, as is nearly the case in aconventional tenter.

The method of a preferred embodiment of the present invention isillustrated by way of the parabolic trajectory example. First theenvelope is chosen as the uniaxial constraint. The TDDR is shown toequal the square of the reciprocal of the cosine of the divergenceangle. The TDDR is equal to the square of the tangent of the divergenceangle plus unity. This allows direct substitution of the left-hand sideof the equation by a function of TDDR only. The equation can then beanalytically integrated to discover the result, equation 1.

In another preferred embodiment, let the extent of uniaxial character U,be defined by a simple ratio according to equation 3:U=(1/MDDR−1)/(TDDR^(1/2)−1)The state U=1 meets the uniaxial condition. States of U between zero andunity represent intermediate states in which some level of MD tensionwill develop. States near unity are nearly or substantially uniaxial.States of U greater than unity represent various levels ofover-relaxing. These over-relaxed states effect an MD compression fromthe boundary edge. If the level of MD compression is sufficient for thegeometry and material stiffness, the film will buckle or wrinkle.

The following discussion is also useful in understanding what is meantby a preferred “substantially” uniaxially drawn film in the context ofthe present invention. One class of preferred curves within an envelopeof final film acceptability are those that maintain the extent ofuniaxial character above a desired threshold value throughout the courseof the draw, since in many final film applications, a final extent ofuniaxial character less than unity can provide acceptable performance.For example, the extent of uniaxial character of 0.85 is sufficient inmany cases to provide an index of refraction difference between the MDand ND directions in polyester systems comprising polyethylenenaphthalate of 0.02 or less at 633 nm for mono-axially transverse drawnfilms. For some polyester systems, such as polyethylene terephthalate, alower U value of 0.80 or even 0.75 may be acceptable, e.g. because oflower intrinsic differences in refractive indices in non-substantiallyuniaxially drawn films. When a specific value of U is chosen, Equation 3provides a specific relationship between MDDR and TDDR which, whencoupled with the aforementioned algorithm or method, specifies a broaderclass of boundary trajectories that also includes the parabolictrajectories as a limiting case when U approaches unity. Trajectoriesthat exhibit values of U below unity for at least a final portion of thedraw are referred to herein as sub-parabolic trajectories.

The condition of constant U less than unity may be approximated by apreferred class of in-plane “sub-parabolic” trajectories in which theparabolic trajectory of equation 1 is used with smaller initialeffective web widths. If x₁ is still taken as the true effective halfwidth of the drawable central portion of the web after effectivegripping (i.e. the initial width minus the selvages held by the gripperswhich is the initial half distance between opposing boundarytrajectories), then this class of trajectories is described by equation4:(x+c)/(x ₁ +c)=(¼) (y/(x ₁ +c))²+1where “c” is a positive offset distance. This class of trajectories is apreferred approximation to constant U for TDDR under 8.

Still another class of boundary trajectories may be preferred insuppressing residual wrinkles. Because the uniaxial condition in theabsence of shear provides a principal MD stress of zero, it isanticipated, using finite strain analysis, that the principal MD stresswill actually go into slight compression under these conditions. Usingfinite strain analysis and a Neo-Hookean elastic solid constitutiveequation, it is discovered that a suitable criterion for preventingcompressive stresses may optionally be given by equation 5:((TDDR)(MDDR))⁻⁴+((TDDR)(MDDR))²−((TDDR)⁻²−(MDDR)⁻²−sin²(θ)((TDDR)(MDDR))⁻²=0MDDR is the cosine of the divergence angle. This optional method of thepresent invention then specifies this class of boundary trajectories.

The class of trajectories described above are illustrative and shouldnot be construed as limiting. A host of trajectory classes areconsidered to lie within the scope of the present invention. The presentinvention preferably encompasses all nearly uniaxial boundarytrajectories comprising a minimum value of U of about 0.70, morepreferably approximately 0.75, still more preferably about 0.80 and evenmore preferably about 0.85. The minimum U constraint may be applied overa final portion of the draw defined by a critical TDDR preferably ofabout 2.5, still more preferably about 2.0 and more preferably about1.5. Above a critical TDDR, certain materials, e.g. certain monolithicand multilayer films comprising orientable and birefringent polyesters,may begin to lose their elasticity or capability of snap back, e.g.because of the development of structure such as strain-inducedcrystallinity. The TDDR may coincide with a variety of material andprocess (e.g. temperature and strain rate) specific events such as thecritical TDDR for the onset of strain-induced crystallization. Theminimum value of U above such a critical TDDR could relate to an amountof non-uniaxial character set into the final film. As discussed above,it may be preferred to introduce a small level of MD tension into thefilm to suppress wrinkling. Preferably, the amount of such MD tensionincreases with decreasing U.

It may be preferred to increase the tension as the draw proceeds. Forexample, a smaller value of U earlier in the draw may tend to set morenon-uniaxial character into the final film. Thus it may be advantageousto combine the attribute of various trajectory classes into compositetrajectories. For example, the parabolic trajectory may be preferred inthe earlier portions of the draw, while the later portions of the drawmay converge on a trajectory of the more expanded parabola of equation 4or the trajectory class of equation 5. In another arrangement, U may betaken as a non-increasing function with TDDR, as opposed to a prior artdisk orienter that decreases U with TDDR.

The parabolic trajectory assumes a uniform spatial drawing of the film.Good spatial uniformity of the film may be achieved with many polymericsystems with careful control of the crossweb and downweb caliper(thickness) distribution of the initial, undrawn film or web, coupledwith the careful control of the temperature distribution at the start ofand during the draw. For example, a uniform temperature distributionacross the film initially and during draw on a film of initially uniformcaliper should suffice in most cases. Many polymeric systems areparticularly sensitive to non-uniformities and will draw in anon-uniform fashion if caliper and temperature uniformity areinadequate. For example, polypropylenes tend to “line draw” undermono-axial drawing. Certain polyesters, notably polyethylenenaphthalate, are also very sensitive.

The invention includes means for gripping the film, preferably the edgesof the film. Preferably, the film is sandwiched between gripper faces ona mechanical clip assembly. The effective edge of the gripper face wherethe film is no longer effectively held defines the edge of the centralportion of the film that will be drawn. This gripper edge defines aboundary edge for the drawing film. The motion of the gripper may alsodefine a boundary trajectory that is, at least in part, responsible forthe motion and drawing of the film (while other effects, e.g., downwebtension and take-up devices, may account for the rest of the motion anddrawing.). Preferably, although not required, the gripper face edges aredesigned so that the center of the edge measured along one clipinstantaneously follows the tangent of a chain riding along a rail orinside a channel cut into the rail. The boundary trajectory may also bedefined by the rail when offset of the gripper edge face from the railchannel are included. In practice, the effective edge of the gripperface can be somewhat obscured by slight film slippage from or flow outfrom under the faces, but these deviations can be made small. Since thefilm is held by two sets of opposing grippers mounted on pairs of chainsand rails, there are two opposing boundary trajectories. Preferably,these trajectories are mirror images about the MD center line of thedrawing film.

The rails are traditionally formed by a series of straight segmentswhose angle of divergence, e.g. the angle formed between the boundarytrajectories and the direction of film travel (e.g. MD), may beadjusted. Curved trajectories have also been explored.

The means for gripping the film according to the present invention maybe discrete or continuous in nature. FIGS. 7 and 8 illustrate details ofa preferred embodiment of an apparatus for stretching films according toan aspect of the present invention. The gripping means comprise a seriesof tenter clips 70 that afford overall flexibility via segmentation. Thediscrete clips 70 are closely packed and attached to a flexiblestructure such as a chain. The flexible structure rides along or inchannels along the trajectory control device, such as a rail.Strategically placed cams and cam surfaces open and close the tenterclips at desired points. The clip and chain assembly may optionally rideon wheels or bearings or the like. Alternatively, the continuousgripping mechanism may comprise a flexible and continuous mechanism,such as a belt or tread. The flexible and continuous mechanism may nestor ride in a groove or a channel. Alternatively, a variety of otheropposing, multiple belt methods, e.g. as described in U.S. Pat. No.5,517,737 or in European patent Application 0236171 A1 (the entirecontents of each of which are herein incorporated by reference) may beused. These may ride in grooves, or ride over bearings or some othermeans of underlying support allowing motion of the flexible continuousmechanism.

Preferred continuous gripping mechanisms provide the advantage ofcontinuously following the changing boundary trajectories along everyportion of the boundary edge. Segmented, discrete gripping systems arewithin the present invention and tend to only approximate the boundarytrajectory along portions of the film at the boundary edges. Forexample, a tenter clip has a straight clip face edge. The clip ismounted so that the center of this clip face edge remains tangent to theboundary trajectory, e.g. tangent to the tenter rail, throughout thecourse of travel and draw. This means that the film gripped at thecenter does follow the boundary trajectory; however, the rest of thefilm gripped along the rest of the clip is constrained to a pathdeviating from the boundary trajectory, unless the boundary trajectoryis straight. The film, at the edge, gripped by single clip wouldotherwise tend to exhibit the divergence angle at the clip center alongthe whole distance of the clip. As a result, film upstream of the clipface edge center would tend to have too large of a divergence anglerelative to the intended boundary trajectories, while film downstream ofthe clip face edge center would tend to have too small of a divergenceangle relative to the intended boundary trajectories. In accordance withthe present invention, a small MD fluctuation in the film properties anduniaxial characteristics may develop. In practice, these MD fluctuationsmay be maintained small by using short enough clips for a given device.For example, the length of a clip face edge may preferably be no morethat one-half, and more preferably no more than one-quarter, the totalinitial distance between the boundary trajectories. Smaller clips willin general provide better approximations to the boundary trajectoriesand smaller MD fluctuations.

Precise control of the divergence angle actually manifested by thegripper mechanism is a design consideration because it is the divergenceangle that contributes to setting the condition for MDDR compatibilitywith the edge. The interactions of the stress field of the film with theboundaries may also tend to moderate approximation errors at the edgesas one proceeds towards the film MD centerline. It may be useful in somecase to reduce the gripper contact to less than the total length of theclip. For example, the film in between two sequential clips experiencesa condition of under-approximated divergence angle from the leading edgeof the upstream clip to over-approximated divergence angle from thetrailing edge of the downstream clip. A slight relaxation of the contactareas at these edges could reduce sharp variations in MDDR and alleviateundesired stress fields that could cause defects. Loss of gripper actionat a portion of the edge may be carefully balanced to reduce unduescalloping at the unsupported edge.

Optionally, the stretcher apparatus may direct airflow towards or intothe boundary edge, e.g. to have air exhaust through the gripper regionto improve heat transfer to the drawable film near the gripper faces.The apparatus may optionally apply active cooling to the grippermechanism, e.g. to the clips, to maintain good gripping of the grippedportion of the film, e.g. by preventing flow out from under the grippermechanism. The particulars of the active heating and cooling will helpto establish the effective boundary edge region. Typically, it ispreferred to have the boundary edge be reasonably approximated by thegripper face edges or by a small offset from these towards the filmcenterline. However, in some cases, e.g. where there is limited abilityto control the shape of the boundary trajectories, it may be preferredto cool or keep cold a small edge region near the gripper face edges inorder to adjust the effective boundary edge from that dictated by theboundary trajectory. In such cases, reasonable attention to adequatecontrol of the temperature uniformity across the major central portionof the film would be desirable to maintain uniformity of the drawing.

The boundary trajectories may be fixed or adjustable. The underlyingcontrol of the boundary trajectories may be like the rails, a movingsurface or some other means of support for a discrete or continuoussystem. The rails may also be segmented and adjustable in part or as awhole. For example, adjustment of the rails or underlying support for abelt system could be made either at junctions or by physical bending,and by various means.

The driving means can be any number of methods. For example, it can bethe motion of the chain as propelled by gears connected to a drive, orthe motion of the belt by an independent drive or by the motion of theunderlying support, e.g. the disk in a disk orienter.

The means of release can be either a physical release of the selvagesheld by the gripping means or a physical separation of the selvages froma central portion of the drawn film.

FIG. 12 illustrates a preferred take away means 100. The take away meanscomprises belts 104 and wheels 102. The takeaway means may include drivewheel 108 and adjustment arms 106. The takeaway means takes hold of atleast a portion of the released film while preferably preventing damageto the usable portion of the film. The take-away device preferablyprovides a means of support while a heated film F quenches. The takeawaymay also preferably comprise a means for controlling shrinkage in eitherthe TD or MD direction or both. Film leaving the takeaway device istypically wound on rolls for later use. Alternatively, direct convertingmay take place after take away. The take-away means may comprise anynumber of methods including a simple roller system with nips, wrapangles, vacuum assists, and the like. For optical films or those withsensitive coatings, a system comprising a top and bottom masking systemsuch as a cover film lamination system may be included. This wouldprotect the surface from the action of a roller system. Anotherattribute of the take-away system is a method of speed and or MD tensioncontrol so that the film can be removed in a manner compatible with theoutput speed. This take-away system could also be used to pull out anyresidual wrinkles in the film. The wrinkles could be initially pulledout during start up by a temporary increase in the takeaway speed abovethe output speed of the final, released portion of the drawn film, orthe wrinkles could be pulled out by a constant speed above the outputfilm speed during continuous operation.

In the above description, the position of elements has sometimes beendescribed in terms of “upper”, “lower”, “over”, “under”, “right”,“left”, “top” and “bottom”. These terms have been used merely tosimplify the description of the various elements of the invention, suchas those illustrated in the drawings. They should not be understood toplace any limitations on the useful orientation of the elements of thepresent invention.

Accordingly, the present invention should not be considered limited tothe particular examples described above, but rather should be understoodto cover all aspects of the invention as fairly set out in the claims.Various modifications, equivalents, as well as numerous structures towhich the present invention may be applicable will be readily apparentto those of skill in the art to which the present invention is directedupon review of the present specification. The claims are intended tocover such modifications and devices.

1. A stretcher for processing a film, the stretcher comprising: aplurality of clips to hold opposing edge portions of the film; pathsalong which the clips travel, wherein at least a portion of the pathsdefine diverging, substantially parabolic courses; and a drive mechanismto convey the film and clips along a machine direction.
 2. The stretcherof claim 1, wherein the paths are adjustable.
 3. A stretcher forprocessing a film, the stretcher comprising: means for receiving a film;means for grasping edge portions of the film; means for conveying thefilm in a machine direction; and means for moving the opposing edgeportions along diverging, substantially parabolic paths to form astretched film.